RTO exam is conducted by public service commission .there are many post also available in RTO.in which you are interested ,it's depend on you.
Yes.surely you are eligible to become a RTO officer after completing the diploma in mechanical engineering.for that you have to apply for RTO exam and have to qualify the RTO.then you are eligible for RTO.

RTO exam clerk post eligibility
* Candidates should be the citizen of India.
* Candidates should have the age limit within 18 to 37 years.
* Candidates should complete graduate from a recognized institute.

RTO exam Judicial post eligibility

* Candidates should be the citizen of India
* Candidates should have the age limit within 23-35 years.
* Candidates should complete graduate in law from a recognized university

RTO exam sub assistant eligibility

* Candidates should be the citizen of India
* Candidates should have the age more than 34 years
* Candidates should complete graduate from a recognized university
* In particular fields such as mechanical and civil

generally RTO posts 3 posts like ::
*************************
-->> CLERKSHIP POST ::
@@ You need have completed 12th from a reconized board of the state.
@@ You need to be citizen of India.
@@ Your age need to be between 18 to 37 years.

-->> JUDICIAL POST ::
@@ You need have graduation degree from a reconized university of law.
@@ Your age need to be between 19 to 23 years.
@@ You need to be citizen of India.
@@ You need to have knowledge of the state mother toung .

-->> THE SUB ASSISTANT ENGINEER POST::
@@ You need have done graduation in engineering from a reconized university of the state.
@@ You age need to be between 23 to 30 years.